1. Field of the Invention
This invention pertains to automated assembly systems, and more particularly to apparatus for welding individual metal pieces into a completed product.
2. Description of the Prior Art
Modern automotive vehicles include a number of sheet metal workpieces welded into a unitary body. The sheet metal workpieces are usually transported along a continuous path between consecutive work stations. At each work station the workpieces are clamped together in accurate relation to each other by suitable tooling. Then the workpieces are welded by welding robots or the like into a sturdy unitary structure.
Examples of prior equipment for manufacturing motor vehicle subassemblies may be seen in U.S. Pat. Nos. 4,162,387 and 4,256,947. In the prior equipment, sheet metal workpieces comprising a subassembly are supported on and transported between a number of framing stations by pallets. At each framing station, a shot pin device locates the pallet in position at a work station. Side gates on both sides of the work station swing about respective horizontal axes from an inoperative position remote from the workpieces to an operative position proximate the workpieces. When the side gates are in the operative position, tooling on the side gates clamps the workpieces. Then the workpieces are welded together by welding robots. After welding, the side gate tooling unclamps the workpieces, and the side gates swing open to their inoperative position. The shot pin device releases the pallet, and the pallet transports the welded subassembly to the next downstream work station for further processing.
Although the prior automotive body welding system have enjoyed some success, they nevertheless possess a few disadvantages. One disadvantage concerns the stack up of tolerances between the pallets and the tooling on the side gates at the work stations. In the prior systems, the sheet metal workpieces are supported on the pallets, and it is the pallets that are shot pinned to the tooling located at the work station. The tolerances inherent in locating the workpieces on the pallet, in locating the pallet at the work station by the shot pin device, and in locating the various tooling elements on the side gates results in undesirable inaccuracies in the locations of the workpieces relative to the side gate tooling. The fact that prior autobody welding systems can employ 20 to 100 or more different pallets increases the probability of detrimental tolerance build up between the workpieces and the tooling.
Another drawback of prior automotive body welding systems involves the swinging of the side gates between their operative and inoperative positions. In prior equipment, the side gates swing about respective horizontal axes located higher from the factory floor than the pallet and workpieces. Because of the configuration of many automotive vehicle bodies, it is frequently difficult to reach and accurately clamp all the workpieces with the tooling in the side gates. Some systems therefore employ secondary swingable gates, frequently called dump units, that are pivotally connected to the side gates at the ends thereof distal from their pivoting ends. The dump units enable tooling to more easily reach the entire workpieces for accurate clamping. However, the dump units add additional variables to the tooling locations relative to the workpieces. In addition, the dump units add considerable expense and complexity to the framing systems.
Thus, a need exists for improvements in automotive body welding machinery.